Abstract:
In the early colonial history of South Africa, horses played an important role, both in general
transportation and in military operations. Frequent epidemics of African horsesickness (AHS)
in the 18th century therefore severely affected the economy. The first scientific research on the
disease was carried out by Alexander Edington (1892), the first government bacteriologist of
the Cape Colony, who resolved the existing confusion that reigned and established its identity
as a separate disease. Bluetongue (BT) was described for the first time by Duncan Hutcheon in
1880, although it was probably always endemic in wild ruminants and only became a problem
when highly susceptible Merino sheep were introduced to the Cape in the late 18th century.
The filterability of the AHS virus (AHSV) was demonstrated in 1900 by M’Fadyean in London,
and that of the BT virus (BTV) in 1905 by Theiler at Onderstepoort, thus proving the viral
nature of both agents. Theiler developed the first vaccines for both diseases at Onderstepoort.
Both vaccines consisted of infective blood followed by hyper-immune serum, and were
used for many years. Subsequent breakthroughs include the adaptation to propagation and
attenuation in embryonated eggs in the case of BTV and in mouse brains for AHSV. This
was followed by the discovery of multiple serotypes of both viruses, the transmission of
both by Culicoides midges and their eventual replication in cell cultures. Molecular studies
led to the discovery of the segmented double-stranded RNA genomes, thus proving their
genetic relationship and leading to their classification in a genus called Orbivirus. Further
work included the molecular cloning of the genes of all the serotypes of both viruses and
clarification of their relationship to the viral proteins, which led to much improved diagnostic
techniques and eventually to the development of a recombinant vaccine, which unfortunately
has so far been unsuitable for mass production.
Description:
Paper given at the 30th
World Veterinary Congress,
October 2011, Cape Town,
South Africa.